This invention relates to a system for monitoring the air/fuel ratio in an internal combustion engine by using an oxygen sensor of the concentration cell type in the exhaust gas.
In recent internal combustion engines and particularly in automotive engines, it is conventional to control the air/fuel mixing ratio precisely to a predetermined optimum value by performing feedback control. In many cases the target value of the air/fuel ratio under feedback control is a stoichiometric air/fuel ratio. For example, when a so-called three-way catalyst is used in the exhaust system to achieve reduction of NOx and oxidation of CO and HC simultaneously, the air/fuel ratio must be controlled precisely to a stoichiometric ratio because this catalyst exhibits optimum conversion efficiencies in an exhaust gas produced by combustion of a stoichiometric air-fuel mixture.
In current feedback control systems, it is usual to produce a feedback signal indicative of the air/fuel ratio of an air-fuel mixture actually supplied to the engine by sensing the concentration of oxygen in the exhaust gas since there is a determined relationship between the air/fuel ratio in the engine and the oxygen content in the exhaust gas.
As for the device to sense the oxygen concentration in the exhaust gas to thereby monitor the air/fuel ratio in the engine, it is usual to use an oxygen sensor of the concentration cell type having a layer of an oxygen ion conductive solid electrolyte such as zirconia stabilized by calcia or yttria and two electrode layers formed on the outer and inner surfaces of the solid electrolyte layer, respectively. An oxygen sensor of this type generates an electromotive force where there is a difference between the partial pressure of oxygen in the exhaust gas to which the outer electrode layer is exposed and an oxygen partial pressure at the inner electrode layer.
In this field, a recent trend is to miniaturize the oxygen sensor by constructing it as a laminate of thin, film-like layers on a plate-shaped ceramic substrate of very small size and by devising a certain method for producing an oxygen partial pressure of suitable level at the inner electrode layer of the sensor without using an external oxygen source material such as air. In an oxygen sensor of this category the solid electrolyte layer is made microscopically porous and permeable to gas molecules. When this oxygen sensor is disposed in the exhaust gas, an oxygen partial pressure nearly equal to the oxygen partial pressure in the exhaust gas always acts on the outer electrode layer. Furthermore, an oxygen partial pressure is produced at the inner electrode layer too by reason of inward diffusion of the exhaust gas containing some oxygen through the porous solid electrolyte layer. However, the oxygen partial pressure at the inner electrode layer is not always equal to the oxygen partial pressure at the outer electrode layer because the solid electrolyte layer is relatively low in its porosity and, hence, offers some resistance to the diffusion of exhaust gas or oxygen molecules therethrough. Therefore, when a considerable change is produced in the concentration of oxygen in the exhaust gas by a change in the air/fuel ratio in the engine across the stoichiometric ratio, a great difference arises between the oxygen partial pressure at the outer electrode layer of the oxygen sensor and that at the inner electrode layer, so that the output voltage of the oxygen sensor exhibits a sharp change from a high level to a low level, or reversely. Such a change in the output voltage of the oxygen sensor can easily be detected by continuously comparing the output voltage of the oxygen sensor with a suitably predetermined reference voltage. Accordingly an oxygen sensor of this type is suitable for use in a feedback control system aiming at a stoichiometric air/fuel ratio in an internal combustion engine.
However, the accuracy of monitoring of the air/fuel ratio by the above described method is not guaranteed when the engine is not operated in a steady state. For example, during operation of the engine under transitional operating conditions, during acceleration with temporarily increased feed of fuel or during temporary cutoff of the fuel feed, there occurs a considerable rise or fall in an average level of the output voltage of the oxygen sensor, whereas the aforementioned reference voltage remains unchanged. Then there arises a possibility that a change in the actual air/fuel ratio across the stoichiometric ratio does not cause the output voltage of the oxygen sensor to intersect the reference voltage, so that the air/fuel ratio is misjudged.